“When a tree falls in a lonely forest, and no animal is near by to hear it, does it make a sound?

I had always assumed that this puzzling question was asked by the philosopher George Berkeley. However, on closer examination, while this issue does relate closely to his philosophy, it was actually posed in this form by Charles Riborg Mann and George Ransom Twiss.

For some people this question is no puzzle – the answer is obvious. For instance, I once watched the TV programme QI where Stephen Fry expressed his shock and disbelief at the notion that the crash of the falling tree made no sound. While the show’s producer tried to explain the puzzle to him he was outraged at what appeared to him to be nonsense: ‘Of course there’s a sound!’

In fact, as philosophical puzzles go this one is fairly straightforward, and can almost be treated as a question of science. For instance Scientific American answered the puzzle like this:

“Sound is vibration, transmitted to our senses through the mechanism of the ear, and recognized as sound only at our nerve centers. The falling of the tree or any other disturbance will produce vibration of the air. If there be no ears to hear, there will be no sound.”

This seems to me obviously correct: the crash makes no sound when there’s nobody to hear it. But the anger and disbelief of Fry touches on a deeper question: “What is really real?”

Some philosopher’s, like Berkeley, insist that what is really real is the sound of the crash. We know, better than we know anything, what we experience – colours, sounds, sensations – these experiences are what make up reality and they assures us by their very vitality that they are real. This kind of perspective is sometimes called idealism – because it locates ultimate reality in those mental experiences that provide us with our sense of reality. However, as the poet Knox observes, idealism also seems ‘ exceedingly odd’:

There once was a man who said ‘God
Must think it exceedingly odd
If he finds that this tree
Continues to be
When there’s no one about in the Quad.’
[R A Knox]

Knox’s poem reflects the conflict between idealism and our common-sense. We may concede that the sound cannot be heard. We may even recognise that all the sensory properties we associate with the tree (its colour, its texture etc.) must rely on a sensing being who can sense those properties. But we still want to say “Ah, but behind all that, there must be the real tree the thing we actually do sense.’ We want to insist that there is a reality to the tree that persists even when there is nobody there to sense it.

However this is so odd isn’t it? We are now insisting on a reality which we can only know indirectly through our senses, and yet which is quite distinct from those senses. The real tree is divorced from the sound of the tree crashing, from the feel of the bark on my fingers, from the green leaves and brown trunk. The properties of the tree that that we actually experience (what John Locke called the secondary qualities) seem to be just the results of our interaction with that deeper reality (the primary qualities) of the underlying tree. So from this more materialist perspective, the real tree is the very tree we don’t directly experience, it is the tree that we imagine to exist and which ’causes’ our experience of the tree.

So, is the real tree the tree we actually experience or is it the tree we imagine to exist and which we believe causes our actual experience of the tree?

Things get even stranger as science gets to work in trying to describe what that real tree is actually made of. I can’t keep up with the latest versions of atomic theory, so just deploy the tools of O-Level science, we are told that the tree is made up of biological substances, which can then be described as complex chemical substances, which can then be described as molecules and atoms, which can then be described as tiny elementary particles or forms of energy, and then there is the vast void which contains them. Under scientific analysis the real tree becomes phantasmagorical.

Is the real tree the tree we actually experience or is it the complex reality that science tries to describe to us in terms of its ever-changing models?

We are pulled both ways. We know the tree is there – because we can see it and touch it. Yet somehow these sensory experiences are not the real tree itself – they are merely signs given to us, expressing a complex reality which we imagine to lie behind our experiences.

So which is real: the sound of the tree crashing which we experience directly or the molecules and atoms which we are told exist and which we use to explain why we see the colour brown?

Today materialism is our dogma. There is less room to question whether we really know what exists. Characters like Stephen Fry would laugh to scorn those who wonder whether matter really exists, or who imagine we have souls, spirits or minds. Fry’s materialism is our modern orthodoxy and all the great thinkers of the past are the heretics. Yet it seems to me that this common and everyday materialism is unsustainable. It wants to have its cake and it wants to eat it too: We assert the ultimate reality of things that we do not experience on the basis of signs that lack the very reality we assert. It is like giving someone food, but then claiming that the ultimate reality is the cookery book that describes how the food was cooked.

Berkeley and other idealist, like my old philosophy professor Timothy Sprigge, believed there was a different way of thinking about reality. It is possible to imagine that our experiences themselves are real, if we recognise that God is the organising principle of reality. As this anonymous poet puts it:

Dear Sir, Your astonishment’s odd:
I am always about in the Quad.
And that’s why the tree
Will continue to be
Since observed by Yours faithfully, God.
[Anon]

I am no master of metaphysics and I am not trying to judge between idealism and materialism, or any other metaphysical theory, but I think it a shame that modern thought has become so diminished that people cannot even see the paradoxical strangeness of the human experience of reality.